The CMS Collaboration has published its first paper based on Run 3 parking trigger in Physical Review Letters, reporting ‘A search for the rare decay D⁰→μ+μ−‘. The analysis is based on data collected during 2022–2023 at a center-of-mass energy of 13.6 TeV. The decay D⁰→μ+μ− is a flavor-changing neutral current process that is highly suppressed in the Standard Model, with a predicted branching fraction of order 10-13. Its extreme rarity makes it a sensitive probe to new physics effects in the up-type quark sector, providing a valuable complement to B physics studies.
This study makes use of the inclusive dimuon parking trigger introduced in Run 3, which allows CMS to record a large number of low-momentum dimuon events that would otherwise be rejected by standard trigger selections. To suppress the significant background from random combinations of muons, an advanced machine-learning algorithm is employed. Each D⁰ meson candidate is required to originate from a D*+ meson, an excited state composed of a charm quark and a down antiquark. The reconstructed mass difference between the D*+ and D⁰ mesons is shown in Fig. 1 (right). The mass difference between D*+ and D⁰, Δm = 145.4 MeV, is so small that the pion, with mass of 139.6 MeV, can barely be produced and is thus very soft. It is a very powerful tool to reduce background from fake D⁰.
No significant excess of D⁰ → μ⁺μ⁻ candidates is observed beyond the expected background. The study sets the world’s most stringent upper limit on the branching fraction, BF < 2.4 × 10⁻⁹ at 95% CL, marking a breakthrough milestone for the CMS flavor-physics program. As the first measurement enabled by the Run 3 parking trigger, this work demonstrates the power of advanced trigger strategies and modern analysis techniques to push the frontiers of particle-physics measurements at the LHC.
